The present invention relates to a laser device with a Q switch for generating a higher harmonics, a method of controlling the device, and a laser processing machine comprising the device.
A conventional Q switch laser device will be explained. FIG. 7 shows a configuration of a conventional a harmonic-generating Q switch laser device. The device comprises a laser head 11, a Q switch 12, an excitation light source 13, a radio frequency (RF) driver 14 for the Q switch, a controller 15, an interface 16, a power source 17, and an operation unit 18.
The operation of the device will be explained with referring to FIG. 7. An oscillation condition instruction set through the operation unit 18 such as a personal computer (PC) is sent to the control circuit 15 through the interface 16. The controller 15 interprets the transmitted oscillation condition instruction, generates a control signal according to the instruction, transmits the condition to the excitation light source 13 and RF driver 14 for the Q switch, and determines the oscillation condition of the laser head. At the same time, the controller 15 also controls the alarm from the laser head 11 and a temperature of a nonlinear optical crystal.
An optical operation of the harmonic-generating Q switch laser
An optical operation of the harmonic-generating Q switch laser device will be explained with referring to FIG. 13. FIG. 13 shows an internal structure of the laser head 11 of the harmonic-generating Q switch laser device shown in FIG. 7. The head 11 comprises a reflecting mirror 21, a Q switch element 42, a gain medium 23, an output mirror 24, a condenser lens 25, a nonlinear optical crystal 26, an optical lens 27, a narrow band filter or dichroic mirror 28, and two lenses 25 and 27. The lenses 25, 27 function as a collimator. The device where the nonlinear optical crystal 26 is disposed outside of the mirrors 21, 24 is called an extra cavity system.
The optical operation of this harmonic-generating Q switch laser head will be explained with referring to FIG. 13. When an excitation light enters the gain medium 23, an optical resonation occurs between the reflector mirror 21 and output mirror 24. In this case, when the Q switch element 42 inserted between the mirrors 21 and 24 is turned on, the optical path opens, and the laser oscillates. When the element is turned off, the optical path closes, and the oscillation stops. Thus, a pulse laser oscillation is enabled. The Q switch element 42 is turned on and off by the RF driver 14 for the Q switch, and enables the Q switch laser head to pulse-oscillate. The laser light issued from the output mirror 24 is condensed by the condenser lens 25, and emitted to the nonlinear optical crystal 26. A harmonic laser is generated by the nonlinear optical crystal 26, is collimated by the optical lens 27, and is separated into an IR laser, a fundamental wave and harmonic laser by the narrow band filter or dichroic mirror 28. The harmonic laser is used for a processing machine.
For example, the power PSHG of a harmonic laser of second harmonic generation (SHG) is expressed in the following formula.
xe2x80x83PSHG=deffxc2x7Ieffxc2x7(PIR)2/A
where deff is a nonlinear constant, a constant determined by the type of the nonlinear optical crystal 26, the larger constant, a higher conversion efficiency; leff is an effective length, a length in which the nonlinear optical crystal 26 has a nonlinear effect; PIR is the power of the fundamental wave; and A is a laser beam area.
Under this relationship, reducing the beam area A and reinforcing the fundamental wave power PIR into the nonlinear optical crystal 26 increases the harmonic laser output. In order to increase the power, the beam put into the nonlinear optical crystal 26 is condensed.
However, the nonlinear optical crystal 26 has a limited light strength, i.e., the crystal is broken when a light exceeding a damage threshold is input. Even if the light is not reaching the damage threshold, a light approaching the damage threshold affects a life of the nonlinear optical crystal 26.
The optical characteristic of the harmonic-generating Q switch laser and a breakdown of the nonlinear optical crystal will be explained with referring to FIG. 9. FIG. 9 shows the oscillation characteristic of the Q switch laser. In a simplest method of operating the Q switch laser device, an arc current of a lamp or current of a laser diode (LD) is maintained in constant, and accordingly, the excitation light power is kept constant, and the Q switch is turned on and off. However, since the excitation light enters the gain medium 23 for a long period while the Q switch closes before an oscillation is started, a gain accumulated in the gain medium 23 excesses a specified value. Therefore, when a gate signal for starting an oscillation is turned on, a giant pulse is generated at a first shot or several shots depending on cases. This giant pulse may destroy the nonlinear optical crystal 26 or shorten the life of the crystal.
To avoid generating such a giant pulse, the controller of the Q switch laser device may have a first pulse suppression (FPS) function.
Referring to FIG. 10, the FPS function of the Q switch laser device will be explained. In order to suppress the giant pulse in the first shot or several shots when the gate signal is turned on upon starting the oscillation, the excitation light into the gain medium 23 is weakened to such an extent as to maintain a specified gain while the Q switch is closed for a long time.
In this method, the laser gain accumulated in the gain medium 23 is prevented from getting excessive. Since the maximum excitation light power is limited, the gradient of building-up the laser gain is limited. The period until the specified laser gain is accumulated depends on the interval of the pulse train following the first pulse, that is, a pulse frequency. Therefore, whenever the pulse frequency changes, the condition of the FPS function such as the laser gain retention current xcex94I and retention period xcex94T must be adjusted.
In other method of realizing the FPS function, the rise time of the Q switch may be delayed, but it is similarly needed to set the condition of the FPS function for each pulse frequency.
FIG. 11 shows an optical system of a laser processing machine having the harmonic-generating Q switch laser device. The processing machine comprises a harmonic-generating Q switch laser device 31, a collimator lens 32, a mask changer 33, a bend mirror 34, a galvanoscanner 35, a scanner lens 36, and a working table 37.
The laser emitted from the harmonic-generating Q switch laser device 31 have the beam diameter optimized by the collimator 32, and is emitted to the mask on the mask changer 33. A portion of the emitted laser passes through the mask, and is condensed at a specified position through the scanner lens 36 by the galvanoscanner 35 through the bend mirror 34, and processes the work fixed on the table 37.
While processing the work, generally, a pulse train of a specified frequency is needed, a long pause period is necessary in order to convey the work, and the pulse oscillation and pause period are repeated.
FIG. 12 shows an output characteristic of the harmonic-generating Q switch laser device. A low pulse frequency increases the pulse energy, and increases a possibility of damaging the nonlinear optical crystal 26 or shortening of the life of the crystal.
Therefore, a long pause period longer in order to convey the work makes the gain be accumulated excessively in the gain medium 23, and generates the giant pulse, which damages the nonlinear optical crystal 26 or shorten the life of the crystal, and thereby reduces the reliability.
To avoid such circumstances, the harmonic-generating Q switch laser device 31 generally incorporated in the laser processing machine has the FPS function as mentioned above.
Particularly a fine processing, the pulse frequency is required to change on the way. In this case, the FPS function is suspended, or even if the FPS function is employed for safety, the period for sending the condition setting instruction of the FPS function is required, and makes an entire processing time longer.
Besides, an unstable pulse processes the work uniformly.
Emitting the giant pulse or first pulse to a dummy target other than the work. the galvanoscanner 35 installed in the processing machine. may eliminates an affect of the pulse to the work. That, however, does not protect the nonlinear optical crystal 26, and the reliability of the device is still not improved.
A laser device and a method of controlling the device are provided for protecting a nonlinear optical crystal without a loss of a cycle time, and for assuring a reliability of a laser processing machine. And a laser processing machine having the device and a processing method using the machine are provided.
The laser device generates a laser pulse train formed of a sequence of laser pulses. The laser device comprises an output mirror, a reflector mirror, a gain medium disposed between the output mirror and reflector mirror for accumulating the laser gain, a Q switch disposed between the output mirror and the reflector mirror for turning on and off a laser oscillation by the output mirror, reflector mirror and gain medium, a nonlinear optical crystal for generating a harmonic laser by recieving a fundamental wave laser by the laser oscillation. During a first pause period before the laser pulse train, the laser oscillation is turned on by the Q switch, and during a second pause period before each laser pulse, the laser oscillation is turned off by the Q switch.